/**
 * @file rank/elementary/luogu1720/main.cpp
 * @brief
 * @see
 * @author Ruiming Guo (guoruiming@stu.scu.edu.cn)
 * @copyright 2022
 * @date 2022/7/7 14:54:48
 **/

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef vector<int> vi;
double sqrt5;
int main() {
  // High rating and good luck!
  sqrt5 = sqrt(5);
  int n;
  cin >> n;
  double ans = (pow((1 + sqrt5) / 2, n) - pow((1 - sqrt5) / 2, n)) / sqrt5;
  printf("%.2lf\n", ans);
  return 0;
}
